1. Field of the Invention
The present invention relates to methods for producing thin-film EL devices that utilize the phenomenon of electroluminescence resulting from the application of an alternating electric field, and particularly it relates to methods of annealing treatment to improve the luminescence of thin-film EL devices, and to apparatuses for producing thin-film EL devices by the above-mentioned annealing treatment.
2. Description of the Related Art
Thin-film EL (electroluminescent) devices may be employed as all solid-state flat panel-type display devices, and because they offer more excellent characteristics than liquid crystal display, such as high contrast and visibility, much research and development is being carried out with the aim of making them practically useful. Research and development is being carried out, for example, for their use in providing color displays. Also, thin-film EL devices capable of providing orange-and-black displays are being satisfactorily used as the display means in FA (factory automation) and OA (office automation) systems.
Double-insulated thin-film EL devices constitute an example of thin-film EL devices whose research and development are presently being promoted for the purpose of practical use. These devices are constructed by laminating on one surface of a translucent substrate realized by, for example, glass, a lower electrode realized by a transparent electrode such as ITO (indium tin oxide), a lower insulating layer, an EL layer, an upper insulating layer and an upper electrode realized by Al (aluminum) or the like, in that order.
The EL layer comprises a host material and luminescence centers incorporated into the host material. When an electric field is generated by applying an alternating voltage between the lower and upper electrodes, the free electrons in the EL layer that have been acknowledged by the electric field collide with the luminescence centers, thus exciting them. The excited luminescence centers then produce the phenomenon of electroluminescence when they return to a stable energy level (ground state). Consequently, it is possible to obtain a display image by combining states of luminescence and non-luminescence through control of the alternating voltage applied to the electrodes.
FIG. 20 is a schematic cross-sectional view of the construction of an annealing apparatus 1 used to produce a conventional thin-film EL device. The EL layer of the thin-film EL device is usually prepared by electron beam deposition, sputtering, CVD (chemical vapor deposition), or the like. Because the EL layer is formed on a lower insulating layer made of a different material from an EL layer material, the crystallinity of the host material is impaired, non-emissive centers are formed in the host material, and the crystal field of the host material is disturbed. In addition, the distribution of luminescence centers in the host material is not uniform, but rather there exist regions of high and low density of luminescence centers, and those regions of high density cause disturbance of the crystal field of the host material. As a result, the flow of electrons which are to excite the luminescence centers is impeded in the regions of high density of luminescence centers, while in the regions of low density of luminescence centers the electrons meet the luminescence centers with less frequency; therefore the excitation efficiency is lowered and the luminance is reduced. The luminance is particularly reduced in the regions of high density of luminescence centers.
In order to prevent this reduction in the luminance, annealing treatment is performed on the EL layer prepared by one of the methods mentioned above, i.e. electron beam deposition, sputtering or CVD. The conventional annealing apparatus 1 shown in the figure has a construction which includes a stage 3 which holds a plurality of thin-film EL devices 2 which are to be treated, a base 4 on which the stage 3 is anchored, a housing 6 on the surface of the base 4 on which the stage 3 is anchored, to create a space 5 within which are situated the stage 3 and the plurality of thin-film EL devices 2 held by the stage 3, a heater 7 for heating the space 5, and a pump 8 for depressurization of the space 5.
FIG. 21 is a flow diagram showing the steps of conventional annealing treatment using the above-mentioned annealing apparatus 1. In step cl, the plurality of thin-film EL devices 2 are placed in the stage 3 anchored to the base 4, and the housing 6 is mounted over the base 4. In step c2, the space 5 in which the stage 3 and the plurality of thin-film EL devices 2 are situated is depressurized by the pump 8 which is, for example, an oil diffusion pump or oil rotary pump. The depressurization is carried out to a degree of vacuum of, for example, 10.sup.-4 Pa or lower.
In step c3, the space 5 is heated by the heater 7. The heating is carried out, for example, at a heat-elevating rate of from 10.degree. to 20.degree. C. per minute to 600.degree. C. This causes heating of the plurality of thin-film EL devices 2 situated in the space 5. In step c4, the space 5 is kept at a prescribed temperature for, as an example, 1 to 2 hours. Intermittent heating of the space 5 by the heater 7 keeps the plurality of thin-film EL devices 2 at the prescribed temperature. In step c5, the space 5 is cooled. It is cooled, for example, naturally by allowing to stand after termination of the heating by the heater 7. The plurality of thin-film EL devices 2 are cooled in this manner.
This annealing treatment rearranges the molecules of the host material and thus improves its crystallinity. In addition, the luminescence centers are diffused in the host material to improve the uniformity of their distribution therein. Since the crystallinity of the host material is improved, there are fewer non-emissive centers and there is less disturbance of the crystal field of the host material. This gives greater freedom of flow to the free electrons which are to excite the luminescence centers. In addition, the greater uniformity of distribution of the luminescence centers reduces the number of regions of high density of luminescence centers which disturb the crystal field of the host material. This increases the frequency with which the free electrons meet the luminescence centers. Consequently, the excitation efficiency of the luminescence centers improves and the luminance increases. Annealing treatment by which this effect is achieved is indispensable during the production process for thin-film EL devices in order to obtain excellent luminescent properties. Furthermore, a higher luminance is generally obtained with treatment at higher temperatures.
The annealing treatment described above is disclosed in, for example, Japanese Patent Application Publication SHO 52-10358. Furthermore, Japanese Patent Application Disclosure HEI 3-141584 discloses an example of forming an Si layer on a lower insulating layer and forming an EL layer over the Si layer. Improvement in the crystallinity of this Si layer is attempted by annealing treatment after depositing the St. The EL layer formed over the highly crystalline Si layer has excellent crystallinity. Laser light or an electron beam is used for this annealing treatment.
Furthermore, in Japanese Patent Application Disclosure HEI 5-159878 there is disclosed an example of annealing treatment which involves heating first by irradiation with light which includes rays in the absorption wavelength band of the luminescence centers, and then by irradiation with light which includes rays in the absorption wavelength band of the host material. The irradiating light used is laser light, and the wavelength band is selected within a range of 100 to 750 nm. Moreover, Japanese Patent Application Disclosure HEI 5-251182 discloses an example of annealing treatment in an inert gas atmosphere.
The disclosed examples mentioned above relate to annealing treatment of thin-film EL devices, but other examples of annealing treatment of silicon thin-films formed on glass substrates have been disclosed, particularly in Japanese Patent Application Disclosure HEI 2-275622. According to this document, heating silicon thin-films by irradiation with light lacking the absorption wavelength component of their glass substrates makes it possible to perform annealing treatment on silicon thin-films in a short time without heat deformation of the glass substrates.
In the case of the examples in Japanese Patent Application Publication SHO 52-10358 and Japanese Patent Application Disclosure HEI 5-251182 mentioned above, time is required for depressurization, heating, maintenance of heating and cooling, and the time for each of these is long. Thus, the long annealing treatment time lowers the productivity of the thin-film EL devices.
Also, in the case of Japanese Patent Application Disclosure HEI 3-141584, since a Si layer is formed by depositing of Si followed by annealing treatment of the Si prior to formation of the EL layer, the step of formation of the Si layer is necessary. Furthermore, because the method uses laser light or an electron beam, which have minute irradiating areas, EL layers with large areas require long times for annealing treatment, and thus the productivity of the thin-film EL devices is lowered.
Furthermore, in the case of Japanese Patent Application Disclosure HEI 5-159878, a two-stage annealing treatment is necessary and laser light is used, and thus the productivity of the thin-film EL devices is lowered for the same reasons mentioned above.
Moreover, the case of Japanese Patent Application Disclosure HEI 2-275622 involves annealing treatment of silicon thin-films formed on glass substrates, not of thin-film EL devices, and therefore the crystallinity of EL layers is not necessarily improved by the annealing treatment disclosed in this document. In addition, EL layers of thin-film EL devices are formed on translucent substrates realized by, for example, glass, and therefore the annealing treatment is performed on the translucent substrates as well. Consequently, there is risk of deformation of the translucent substrates when they are kept at high temperatures for prolonged periods.